Iran's supreme leader on Friday banned the import of American and British-produced vaccines against Covid-19, saying they were "completely untrustworthy".
"Importing vaccines made in the US or the UK is prohibited," Ayatollah Ali Khamenei said in a tweet, accompanied by the hashtag #CoronaVaccine.
"It's not unlikely they would want to contaminate other nations," he added.
The Islamic republic has reported more than 1.2 million cases of the novel coronavirus, which have caused over 56,000 deaths.
It has accused arch-enemy the US of hampering its access to vaccines through a tough sanctions regime.
While food and medicine are technically exempt from the measures, international banks tend to reject transactions involving Iran.
Iran's President Hassan Rouhani said last month that Washington had demanded that Tehran pay for the drugs through US banks, adding that he had feared the United States would seize the money.
Khamenei's edict bans imports of the Pfizer-BioNTech, Moderna and AstraZeneca-Oxford anti-Covid 19 jabs.
Responding to Khamenei's move, the World Health Organization's emergencies director Michael Ryan stressed that the world body had made repeated calls to "not politicise this virus".
"Please, let us not politicise this vaccine either," he said at a news conference in Geneva.
The pandemic came on top of months of escalating tensions between Tehran and Washington, after US President Donald Trump unilaterally withdrew from a 2015 treaty on Iran's nuclear programme and reimposed suffocating sanctions.
In March last year, Khamenei rejected an offer by Trump to help with the pandemic, saying he did not trust the intentions of the US, Iran's "most vicious" enemy.
Iran last month launched clinical trials of its own vaccine developed inside the Islamic republic, the Middle Eastern country hardest hit by the pandemic.
The Iranian Red Crescent said Friday that US-based Iranian scientists had been planning to send 150,000 doses of the Pfizer-BioNTech vaccine to Iran, but that the delivery had been cancelled following Khamenei's comments.
"A million doses of the vaccine were meant to have been delivered to the Red Crescent" via a third country, it said in a statement widely reported in local media.
Khamenei claimed in a televised address Friday that "if the Americans were able to produce" a trustworthy vaccine, "the coronavirus catastrophe wouldn't have happened in their country."
Khamenei also tweeted that "given our experience with France's HIV-tainted blood supplies, French vaccines aren't trustworthy either".
That was a reference to a scandal in the 1980s in which blood infected with HIV was distributed in France, and later abroad, even after the government became aware of the problem.
Hundreds of people in Iran were among those infected.
France's then-prime minister Laurent Fabius was charged with manslaughter, but acquitted in 1999, while his health minister was convicted but never punished.